Elizabeth Smart's alleged kidnapper collapsed at his trial in Salt Lake City on Tuesday morning, possibly from a seizure, and was taken to hospital.

As the judge entered the courtroom to begin proceedings, Brian David Mitchell, who entered the courtroom singing "O Holy Night," dropped his head, was silent for about two minutes, then moaned loudly and collapsed, local TV station KSL reports.

"Judge, I think he's having a seizure," one of Mitchell's attorneys said.

Mitchell was treated at the scene, while remaining shackled, and then taken to hospital, looking alert but wearing an oxygen mask.

It was not confirmed that he had a seizure, or what other medical problem he might be suffering from. Mitchell has exhibited odd behavior throughout the trial, and has even been kicked out of court for singing.

His stepdaughter, Rebecca Woodridge, said Mitchell has been suffering seizures in jail for about a year but has refused medical treatment. "This isn't something he's making up," she told reporters outside the courthouse. "This is real."

Mitchell is standing trial for the nine-month abduction of Smart when she was 14. Charges against him include kidnapping and sexual assault. During her testimony Smart mentioned that Mitchell had experienced a seizure while they were in California. "He was in the middle of raping me and he experienced a seizure,” she testified. She said it was the only time she had seen Mitchell have a seizure.